Treating of aluminum and aluminum alloy surfaces



2,785,098 TREATINGOF AL AND ALUMINUM ALLoY sunrxcns George L Cieveland Heights, and Martin 4 Claims. (Cl. 148-617) invention relates to the treatment of aluminum an'daluminu'rn alloy surfaces to produce thereon coatings possessing improved resistance to corrosion and abrasion. More particularly, it relates to the production of coatings which are fast to light and water and which may be colored or black or white.

Many processes are known for the production of decorativecoatings on aluminum. One such typical process is described in U. S. Patents 1,965,269 and 1,988,012, wherein the procedure disclosed involves first the production of an absorbent oxide film on the aluminum surface, then the deposition therein of a substance which contributes the desired metal ion and finally a subsequent oxidation or reduction of the deposited material to produ'ce an insoluble pigment in the pores of the oxide coatiiig. Another prior art approachis disclosed in U. S. Patents 2,018,388 and 2,022,798; in which an absorbent oxide coating is first produced on the surface to be colo'red and the surface is subsequently treated with two solutions which react metathetically to produce an insoluble inorganic compound in the pores. One outstanding defect in these and other known processes has been the difiiculty associated with the complete removal of any negative ions from the treated surface since the presence of such ions tends to impair the corrosion resistance of the metal of alloy.

It is one object of this invention to provide a method of producing coatings on aluminum or aluminum alloy surfaces in which the coatings are substantially free from any negative ions such as chloride, nitrate or the like.

Another object of this invention is to provide a method by means of which dense and uniform coatings may be obtained which are of any desired color.

Another object of our invention is to provide a simpler method than the aforesaid prior art processes in which the desired pigment is obtained in the pores of an absorbent oxide or hydrated oxide layer by hydrolysis instead of by metathetical or other chemical reactions.

Broadly, our invention comprises in one form first treating the aluminum or aluminum alloy to provide a hard and absorbent oxide or hydrated oxide surface,

form of a metal-organic compound), then hydrolyzing the reagent in situ in the pores of the oxide coating and finally sealing the surface by exposing it to the action of a warm aqueous medium whereby the pigment is retained inside the oxide layer. 7

In an alternative form, after hydrolyzing the reagent, the impregnated surface may be exposed to the action of a material with which it reacts to form an insoluble pigment in the pores of the absorbent layer and a readily separable by-product. V I

Substrates which have been found to be amenable to our invention include aluminum and alloys in which the major component is aluminum, such as those described in Tables I11 and IV on page 793 of the 1948 Handbook published by the American Society for Metals It will therefore be understood that the term aluminum as-herea 2,785,098 Ice Patented Mar. 1957 inafter employed,- is intended to cover not only the metal but also any alloys in which it is the predominantconstituent. V v v The formation of an absorbent oxide or hydrated-oxide film on such materials'is old and well known and may be accomplished by any of several techniquesknown to those skilled in the art. For example, the oxide or hydrated oxide coating may be produced by treating the surface chemically, e. g; by contacting the surface with a solution of an alkali carbonate and a soluble dichromate, as described in Tosternd Patent 1,946,150. Al ternatively, as described in the same patent, the oxide or hydrated oxide may be produced by anodic oxidation of the surface in oxalic acid-or sulfuric acid, or other appropriate liquid medium. Whether chemical or electrochemical means are chosen to produce the adsorbent fil'r'n is immaterial, provided that'a relatively thick film is formed. The thin aluminum oxide which forms naturally when the metal is exposed to air is not of suficient thickness to satisfactorily retain the insoluble inorganic pigment in the amounts found necessary to obtain the benefits of our invention;

After the oxide or hydrated oxide coating of a suitable thickness has been formed on the aluminum, the coated metal is treated to deposit a water-insoluble inorganic pigment within the adsorbent. oxide layer. Essentially this treatment involves contacting the oxide or hydrated oxide layer witha liquid reagent which is adsorbed by'the oxide and then hydfolyzing the reagent to leave the water insoluble pigment in the po'res of the-oxide layer and to form at the same time an alcohol orother organic compound, which'imay 'bETIEflClllY. removed fromithe' surface and whichwill not leave behind any residual negative chloride, nitrate or other ions found to promote corrosion. Reagents which satisfy the above requirements are generally those metal-organic compounds which; when reacted with water, fdrm an organic compound and the desired water insoluble pigment; e; 'g; a metal oxide; A wide variety of metal organic compounds have been found suitable for the practicev of our invention. Particularly attractive results have been obtained with rthe alcoholates of boron, calcium, chromium, cobalt; copper, iron, magnesium, nickel, selenium, 'tellurium, tin titanium, zirconium and oth'er metals. Methyl, ethyhpropyl, isopropyl or higher alcoholates have been successfully used, this choice being dependent to some extent on solubility, stability, and ease of manufacture. The metal of the metal-organic compound is selected primarily on the basis of the color and tone to be achieved in the finished product. For distinctly dense white coatinigon aluminum and aluminum alloys, titanium dioxide or bydrated titanium dioxide produced in the oxidelayer by the hydrolysis of titanium alcoholates is very useful. Other colors are obtained by a suitable choice of indiganic oxide, e. g. green from chromium oxides by hydrolysis of a chromium alcoholate, yellow, reds and even black from iron oxide by hydrolysis of the corresponding iron alcoholate, black from copper oxide, and whit'e from the hydrolysis of calcium, magnesium, zirconium and many other alcoholates; Thus,by selecting an appropriate alcoholate or mixture of alcoholates, it is possible to obtain a wide variety of colors and a wide variety of tones. v H V J 7 When the alcoholate is a liquid, it is sprayedon the metal, or the metal may be dipped in it until the required amount has been adsorbed. In some cases the alcoholate is solid and inust be melted or dissolved, in an appropriate solvent, e. g. an alcohol. In still other instances the metal alcoholate is decomposed by heatingand isin soluble in most solvents. This difiiculty can oftenibe overcome by reacting the metal alcoholate with another 3 metal alcoholate or its ortho ester, to form an alkoxo compound which may be readily dissolved in a suitable solvent and thus employed in liquid form, In thecase of metal organic compounds which are capable of being distilled without substantial decomposition, the substrate, after the oxide of hydrated oxide coating of a suitable thickness has been formed thereon, may be impregnated with the metal-organic compound by heating same and exposing the substrate to the vapors produced.

-In addition to the metal alcoholates and their derivatives above described, other metal-organic compounds which hydrolyze without impairing the corrosion resist- V ance of the coated metal by forming strongly negative ions such as chloride and nitrate have been found useful. One such group of compounds are the metal tetraborohydrides represented by the formula X[B(OR)4ln where X is a metal, R is a methyl or ethyl radical and n is an integer. By way of example, alkaline earth metal tetramethoxyborohydrides such as Ca[B(OCHs)4]2 react with water to produce the borate and an alcohol, the alkaline earth borate being retained in the pores as the insoluble pigment.

The adsorbent oxide or hydrated oxide layer may be processed so as to contain water insoluble inorganic pigments other than oxides, by means of an additional step. For example, if the color desired in the coating is the color of the sulfide of a particular metal, then the oxide is. de-

posited in the adsorbent layer from a suitable metal-or- Ii ganic compound, as above described, and then exposed to a sulfiding agent such as gaseous hydrogen sulfide, either anhydrous or containing moisture or an ammonium sulfide solution, or a solution of another sulfide. In this manner white cadmium oxide may be converted .to a

bright yellow cadmium sulfide.

The final step in the overall process comprises treating the aluminum or aluminum alloys to seal the anodized surface and thus seal the inorganic pigment'inside this hard layer. The sealing may be effected by immersing the aluminum or aluminum alloy in hot Waterfor about 15 minutes or by heating the coated aluminum in a moist atmosphere for between 5 and 30 minutes depending on the temperature.

The following examples willfurther illustrate the practice of our invention: 1

Example 1.- aluminum plates 3.6" x 10 x 0.020"

were caustic etched and anodized for minutes in a 3 percent oxalic acidl percent sodium oxalate bath at C., using a currentdensity of 20 amperes per square foot.

Afterthe voltage had increased from 40 to volts, the anodizing was stopped. The anodized plates were removed from the bath, and were washed and dried. The anodized plates were immersed in tetraisopropyl titanate,

then removed and wiped to remove excess liquid from the surface. The plates were then dipped in a dilute solution 7 of ammonium hydroxide. A dense, white layer was formed in the anodized layer. The plates were then sealed by placing in boiling water for 30 minutes and wiped dry.

Example 2.Similar-plates of aluminum alloy. were anodized in the conventional manner and thereafter immersed in an ethyl alcoholsolution of Cd[Al(OC2l-i5)4] 2. The compound deposited in the porous oxide coating and was converted to yellow cadmium sulfide by treatment a with a solution of an alkali sulfide. The treated plate was washed with cold water and sealed by. placing same in Example 4.--Additional plates of aluminum alloy were anodized as above and then immersed in an ethyl alcohol solution of Cu[Al(OC2I-I5)4]z. The impregnated plates were then dipped in cold water to hydrolize the alcoholate thereby depositing copper oxide in the pores of the oxide coating. The pigment was sealed in place by dipping the plates in hot water for 10 minutes. An attractive dark color was obtained on the plates.

Example 5. Each of the above experiments was repeated with aluminum or aluminum alloy plate material which had been anodized chemically instead of electrolytically. The anodizing procedure consisted of immersion in a boiling solution containing about 0.5 to 7% sodium carbonate and about 0.01 to 1% potassium dichromate. After such a treatment, the plates were thoroughly washed with cold water and then subjected to metal-organic reagents of Examples 1 through 4 with entirely equivalent results.

We claim:

1. A method of forming a permanent colored coating on a metal substrate which is predominately aluminum which comprises forming an absorbent oxide coating on the substrate, depositing in the pores of the absorbent oxide coating a readily hydrolyzed metal-organic compound from the group consisting of metal alcoholates and metal tetraborohydrides having the formula X[B(OR)4]72 where X is a metal, R is a radical from the group consisting of methyl and ethyl radicals, and n is an integer, hydrolyzing the deposited metal-organic 'compound to form as one product a metal compound which is retained in the pores of'the coated substrate and as a second product a readily removable non-corrosive alcohol residue, and finally sealing the treated surface by exposing said surface to the action of an aqueous medium whereby the deposited pigment is retained inside the oxide layer.

2. A method of forming a permanent colored coating on a metal substrate which is predominately aluminum which comprises forming an absorbent .oxide coating on the substrate, depositing in the pores of the absorbent ioxide coating, a readily hydrolyzed alcoholate, hydrolyzing the deposited alcoholate to form as one product a metal compound which is retained in the pores of the V coated substrate and as a second product a readilyremovable non-corrosive alcohol residue, and finally sealing the treated surface by exposing said surfacezto the action of an aqueous medium, whereby the deposited pigment is retained inside the oxide layer.

3. A method of forming a permanent colored coating on a metal substrate which is' predominately aluminum which comprises forming an absorbent oxide coating on the substrate, depositing in the pores of the absorbent oxide coating a readily hydrolyzed metal-organic vconipound from the group consisting of metal alcoholatesand metaltetraborohydrides having the formula X[B(OR)4ln where X is a metal, R is a radical from the group consisting of methyl and ethyl radicals, and n isan integer,

hydrolyzing and sulfidizing the deposited metal organic compound to' form as one product a metal sulfide which is retained in the pores of the coated substrate. and as a second product a readily removable non-corrosive alcohol residue, and'finally sealing the treated surface 'by exposing said surface to the action of an aqueous medium wherebythe deposited pigment is retained inside the oxide la er. 1

it. A method of forming a permanent colored coating on a metal substrate which is predominately aluminum which comprises forming an absorbent oxide coating on the substrate, depositing in the pores of'the absorbent oxide coating a readily hydrolyzed metal-organiccompound from-the group consisting of metal alcoholates and metal tetraborohydrides having the formula XiB(QR) 4] where X is a metal, R is a radical from the group consisting of methyl and ethyl radicals, and n is an. integer, hydrolyzing the deposited metal-organic. compound, to

form as one product a metal compound which is retained in the pores of the coated substrate and as a second product a readily removable non-corrosive alcohol residue, sulfidizing the deposited metal compound by treating same with a sulfidizing agent of the group consisting of alkali sulfides, ammonium sulfides and hydrogen sulfide, washing the substrate and finally sealing the treated surface by exposing said surface to the action of an aqueous medium, whereby the deposited pigment is retained inside the oxide layer.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF FORMING A PERMANENT COLORED COATING ON A METAL SUBSTRATE WHICH IS PREDOMINATELY ALUMINUM WHICH COMPRISES FORMING AN ABSORBENT OXIDE COATING ON THE SUBSTRATE, DEPOSITING IN THE PORES OF THE ABSORBENT OXIDE COATING A READILY HYDROLYZED METAL-ORGANIC COMPOUND FROM THE GROUP CONSISTING OF METAL ALCOHOLATE AND METAL TETRABOROHYDRIDES HAVING THE FORMULA X(B(OR)4)N WHERE X IS A METAL, R IS A RADICAL FROM THE GROUP CONSISTING OF METHYL AND EHTYL RADICALS, AND N IS AN INTEGER, HYDROLYZING THE DEPOSITED METAL-ORGANIC COMPOUND TO FORM AS ONE PRODUCT A METAL COMPOUND WHICH IS RETAINED IN THE PORES OF THE COATED SUBSTRATE AND AS SECOND PRODUCT A READILY REMOVABLE NON-CORROSIVE ALCOHOL RESIDUE, AND FINALLY SEALING THE TREATED SURFACE BY EXPOSING SAID SURFACE TO THE ACTION OF AN AQUEOUS MEDIUM WHEREBY THE DEPOSITED PIGMENT IS RETAINED INSIDE THE OXIDE LAYER. 